Fowlpox virus (FPV) is the archetypal member of the avian poxviruses and the causative agent of pox in poultry (Woodruff, A.M., and E.W. Goodpasture (1931) Am. J. Pathol. 7:209-222; Woodruff, C.E., and E.W. Goodpasture (1929) Am. J. pathol. 5:1-10; Woodruff, C.E., and E.W. Goodpasture (1930) Am. J. Pathol. 6:713-720). The virus particle is brick-shaped with dimensions of 260.times.350 nm and possesses the typical poxvirus structure. An outer membrane system encloses the lateral bodies and the biconcave core containing the viral genome which has been estimated at 200-240.times.10.sup.6 daltons (Gafford, L.G. and C.C. Randall (1967) J. Mol. Biol. 26:303-310).
Pox of birds is prevalent world-wide but is not considered a public health problem since the host-range of the avian poxviruses is limited to birds and excludes mammals (Tripathy, D.N. and G.H. Cunningham (1984) Avian Pox, Chapter 23, pp. 524-534, in Diseases of Poultry, 8th ed. M.S. Hofstad ed.). Chickens of all ages are susceptible to the disease and while mortality is usually low, infection causes a temporary decrease in egg production and a significant reduction in the growth rate of young birds. FPV infection most often occurs by mechanical transmission to injured or lacerated skin although the virus can also be transmitted by mosquitoes (DaMassa, A.J. (1966) Avian Dis. 10:57- 66). After an incubation period of 4 to 10 days, the disease manifests itself as one or a combination of three forms: (1) cutaneous lesions of featherless areas; (2) dipthenic lesions of the mouth; and (3) coryzal lesions of nasal passages (Tripathy, D.N., and C.H. Cunningham (1984) Avian Pox, Chapter 23, pp. 524-534, in Diseases of Poultry, 8th ed. M.S. Hofstad ed.). In uncomplicated infections the disease lasts 3-4 weeks and results in life-long immunity in the bird, a result of both humoral and cell-mediated responses (Tripathy, D.N., and L.E. Hanson (1975) Am. J. Vet. Res. 36:541-544).
Attenuated strains of FPV are currently being used by the poultry industry as vaccines to control the incidence of pox in chickens and turkeys. The live viral vaccine, which results in life-long immunity, is prepared on the chorioallantoic membrane of the chicken embryo or from chicken embryo fibroblast cell cultures. Vaccinations are administered to chicks as young as one day old either orally or by pricking the web-wing (Tripathy, D.N., and C.H. Cunningham (1984) Avian Pox, Chapter 23, pp. 524-534, in Diseases of Poultry, 8th ed. M.S. Hofstad ed.; Mayr, A., and K. Danner (1976) Develop. biol. Standasr. 33:249-259). The FPV vaccine has been used in combination with a vaccine for Marek's Disease Virus to protect against both diseases with a single innoculation (Siccardi, F.J. (1975) Avian Dis. 19:362-365).
Laboratory analyses of FPV have concentrated on the characterization of the growth of the virus in birds, the chorioallantoic membrane (CAM) of developing embryos, and tissue culture cells. Replication in the dermal or follicular epithelium of birds is similar to that on the CAM (Tripathy, D.N., and C.H. Cunningham (1984) Avian Pox, Chapter 23, pp. 524-534, in Diseases of Poultry, 8th ed. M.S. Hofstad ed.). After adsorption, penetration and uncoating of the virus, a host response consisting of hyperplasia and the replication of cellular DNA occurs for the first 72 hours and generally results in a 2.5 fold increase in the number of cells (Cheevers, W.P., and C.C. Randall (1968) Proc. Soc. Exp. Biol. Med. 127:401-405; Cheevers, W.P., D.J. O'Callaghan, and C.C. Randall (1968) J. Virol. 2:421-429). Viral DNA replication which is preceded by early protein synthesis occurs primarily between 60 and 96 hours post-infection and is followed by the synthesis of late proteins. The assembly of infectious virions occurs between 72 and 96 hours (Cheevers, W.P., and C.C. Randall (1968) Proc. Soc. Exp. Biol. Med. 127:401-405; Cheevers, W.P., D.J. O'Callaghan, and C.C. Randall ( 1968) J. Virol. 2:421-429).
The growth of FPV on tissue culture cells has been achieved on chicken embryo fibroblast cells, duck embryo fibroblast cells, and chicken embryo dermal cells (Gafford, L.G., and C.C. Randall (1976) Virology 33:112-120; Baxendale, W. (1971) Vet. Rec. 88:5-10; El-Zein, A., S. Nehme, V. Ghoraib, S. Hasbani, and B. Toth (1974) Avian Dis. 18:495-506). In each case, the viral cycle is similar and appears to be quicker than that defined in birds. In the CED cells DNA replication commences between 12 and 16 hours, and infectious virus particles first appear at 16 hours and continue to increase in number until 48 hours post-infection (Prideaux, C.T., and D.B. Boyle (1987) Arch. Virol. 96:185-199).
Investigations of the organization of the FPV genome have recently been reported by a number of laboratories. The thymidine kinase gene was identified by complementation of a thymidine kinase negative vaccinia virus, and the DNA sequence of this gene has been determined (Boyle, D.B., and B.H. Coupar (1986) J. Gen. Virol. 67:1591-1600; Boyle, D.B., B.H. Coupar, A.J. Gibbs, L.J. Seigman, and G.W. Both (1987) Virology 156:355-365). Importantly, this study demonstrated the functional cross-reactivity of FPV and vaccinia virus promoters. The FPV DNA polymerase gene was identified by amino acid homology to the vaccinia virus polymerase, and the DNA sequence of this gene was also reported (Binns, M.M., L. Stenzler, F.M. Tomley, J. Cambell, and M.E.G. Boursnell (1987) Nucleic Acid Research 16:6563-6573). Twenty-one polypeptides associated with the FPV infectious cycle have been detected by metabolic labeling of infected chicken dermal cells, and a 3.1 kb fragment of the FPV genome which demonstrates nucleic acid homology with the vaccinia virus Hind III J fragment has been identified (Prideaux, C.T., and D.B. Boyle (1987) Arch. Virol. 96:185-199; Drillien, R., Spehner, D., Villeval, D., and J.-P. LeCocq (1987) Virology 160:203-209).
Vaccinia virus, the archetypal member of the orthopox viruses, was employed as a vaccine for the successful worldwide erradication of smallpox. The success of the program is attributable in part to: (1) the high levels of both cellular and humoral immunity achieved in response to infection with vaccinia virus; (2) the stability of the virus; (3) the ease of administration of the vaccine; and (4) the relative safety of the innoculation.
Paoletti et al. have developed a technique known as in vivo recombination (IVR) which allows the insertion by site-specific recombination of foreign DNA into the vaccinia virus genome (U.S. Pat. No. 4,603,112), and has led to the use of vaccinia virus as a eukaryotic expression vector for creating live recombinant vaccines. A number of recombinant vaccinia virus have been created expressing either single or multiple genes encoding specific foreign viral antigens and upon vaccination have been shown to protect against challenge with the correlate pathogens.
Recently, Boyle, D. et al. have disclosed recombinant FPV containing foreign DNA within a nonessential region of the viral genome. International Patent Application PCT/AU87/00323. Vaccinia virus promoters are used to express the DNA in FPV.